Optical semiconductor device and method for manufacturing the same

ABSTRACT

An optical semiconductor device comprising a plurality of semiconductor lasers formed on a single substrate is provided, in which each of said semiconductor lasers emits a laser lights having designed different oscillating wavelength. This optical semiconductor device is provided by maintaining the coupling coefficient of each of said semiconductor lasers at a constant value by adjusting the composition of an optical guide layer or the mask width for the MOVPE growth.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method of manufacturing anoptical semiconductor device, and in particular relates to a collectivemanufacturing method of the optical semiconductor device.

[0003] 2. Background Art

[0004] In order to cope with an increasing demand for communicationsystems, an optical communication system based on the WDM (wavelengthdivisional multiplexing) system is being developed, because it iscapable of expanding the channel capacity of the optical fiber bytransmitting light signals having different wavelengths in one opticalfiber without extending the manufacturing facility. It is a matter ofcourse that this WDM optical communication system requires a lightsource, which emits a plurality of lights having multiple wavelengths.Thus, the problem in this WDM optical communication system is to providea light source laser which can emit a plurality of lights havingmultiple wavelengths.

[0005] Japanese Unexamined Patent Application, First Publication No. Hei10-117040 discloses a collective manufacturing method of a semiconductordevice in which different wavelength DFB (distributed feedback) lasersand different wavelength EA (electro-absorption) modulator integratedDFB lasers are integrated on a semiconductor substrate. In the aboveJapanese patent application, a method is used for providing multipleoscillating frequencies on a single semiconductor substrate by firstforming diffraction gratings having different cycles (pitches) A asshown in FIG. 46 by electron beam exposure and by an etching techniqueand by forming a multiple layered structure including active layers(light absorbing layers) having multiple band-gap wavelengthscorresponding to the oscillation wavelengths by a selective MOVPE(metal-organic vapor phase epitaxy) growth method. In this method, sincethe oscillation wavelength can be made to coincide to some extent withthe band-gap wavelength of the laser active layer, the threshold for thelaser oscillation or the homogeneity of the oscillation efficiency canbe maintained comparatively consistent.

[0006] However, when the active layers (and light absorbing layers) areproduced by selective MOVPE growth, the band-gap wavelength of the lightguide layer formed on the diffraction gratings and the thickness of theactive layers change. When the band-gap wavelength of the light guidelayer changes, the absolute value of the refractive index of the layerson the diffraction gratings changes, which results in changing theperiodic change of the refractive index by the diffraction grating. Whenthe thickness of the active layer changes, a light confinement factor inthe active layer changes, which results in changing the light intensityof the diffraction grating region. The variation of the periodic changeof the refractive index of the guide layer by the diffraction gratingand the light intensity of the diffraction grating region are parametersrelated to a coupling coefficient κ, the coupling coefficient κ changesas the oscillation wavelength changes in the collective formation of themultiple wavelength laser in which variations occur in both of theperiodic change and the light intensity change.

[0007] The technique disclosed in the above-described Japanese patentapplication is to broaden the mask width of a pair of stripe-like masksfor the selective growth, in order to lengthen the oscillationwavelength. However, there are two factors affecting the couplingcoefficient κ: (1) the absolute value of the refractive index of thelight guide layer becomes large because the band-gap wavelength of thelight guide layer on the diffraction grating increases to a longerwavelength; and (2) since the thickness of the active layer increases,the light confinement coefficient in the active region increases and asa result, the light intensity in the diffraction grating regiondecreases. The effect of the above item (1) has an action to increasethe coupling coefficient κ and the effect of the above item (2) has anaction to decrease the coupling coefficient κ, so that the relationshipbetween the oscillation wavelength (or the selection growth mask width)and the coupling coefficient κ changes depending upon the ratio of themagnitudes of the above effects of (1) and (2).

[0008] The ratio of the magnitudes of the above effects (1) and (2) isdependent on the MOVPE apparatus for growing the crystal or the growthconditions. The coupling coefficient κ is a parameter related to theoscillation threshold value or the light emission efficiency of the DFBlaser, the single longitudinal mode yield, or the long distancetransmission characteristics. Thus, if the crystal is not grownhomogeneously, the manufacturing yield of the elements is decreased.

[0009] There is a trade-off between the longitudinal single mode yieldand the effect of the coupling coefficient κ on the long distancetransmission characteristic, and the provision of the optimizedhomogeneous κ value is an important factor for obtaining a high finalyield.

[0010] Therefore, the first problem is that the longitudinal single modeoscillation yield decreases as a result of the heterogeneity generatedin the laser oscillation threshold currents and in the light emittingefficiencies at various wavelengths. This is caused due to the fact thatthe light emitting elements emitting lights with different wavelengthsfrom each other have different coupling coefficients.

[0011] The second problem is that, when each laser light is transmittedfor a long distance, the transmission characteristic yield for a laserlight changes depending on the wavelength of the laser light. Since thecoupling coefficient of each laser oscillating element changes, thewavelength variation (wavelength chirp) for each laser oscillatingwavelength by residual reflection at each end surface changes for eachlaser element.

[0012] The object of the present invention is to suppress the dispersionof the coupling coefficients of the DFB laser portion, which is usuallycaused at the time of collective forming of the different wavelength DFBlaser or the different wavelength EA modulator integrated DFB laser on asemiconductor substrate, and the object of the present invention furtherextends to homogenization of the laser oscillation threshold value, thelight emitting efficiency, and the long distance transmissioncharacteristics, and to the improvement of the longitudinal single modeoscillation yield.

[0013] The object of the present method for manufacturing the opticalsemiconductor device is to provide a method which is capable of solvingthe problems occurring in the conventional methods for collectivelyforming the different wavelength DFB laser. That is, the object of thepresent invention is to provide a method for manufacturing the differentwavelength DFB laser or the different wavelength DFB laser integratedelement, which is capable of maintaining the coupling coefficient κ at aconstant value, even if the oscillating wavelength differs.

SUMMARY OF THE INVENTION

[0014] The present invention has been carried out to overcome theabove-described problems and the following technical constitution hasbeen obtained.

[0015] The present invention provides an optical semiconductor devicecomprising a plurality of semiconductor lasers formed on a singlesubstrate, wherein each of said semiconductor lasers emits laser lightshaving oscillating wavelengths differing from each other by differentcycles of a plurality of diffraction gratings, wherein the compositionof an optical guide layer in contact with one of said diffractiongratings is determined such that the coupling coefficient of each ofsaid semiconductor lasers is maintained at a constant value.

[0016] The present invention also provides an optical semiconductordevice comprising a plurality of semiconductor lasers formed on a singlesubstrate, wherein each of said plurality of semiconductor lasers emitslongitudinal single mode laser lights having different oscillatingwavelengths due to a distributed feedback operation of a periodic changeof the refractive index in the semiconductor lasers, and wherein each ofsaid plurality of semiconductor lasers have the same couplingcoefficient by being provided with a diffraction grating embeddedsemiconductor layer each having a refractive index corresponding to theoscillating wavelength.

[0017] In the above optical semiconductor device, each of said pluralityof semiconductor lasers comprises a diffraction grating embeddedsemiconductor layer made of InGaAsP having a band-gap wavelength(energy) corresponding to the oscillating wavelength thereof, and eachof said semiconductor lasers is a distributed feedback semiconductorlaser.

[0018] In the above optical semiconductor device, an optical modulatoris monolithically integrated with said semiconductor laser.

[0019] In the above optical semiconductor device comprising a pluralityof semiconductor lasers comprising an InGaAsP guide layer formed on orbelow said diffraction gratings, a multi-quantum well layer, and an InPclad layer on an InP substrate, and which emit laser lights havingdifferent wavelengths determined by the cycle of said diffractiongratings, the refractive index of said guide layer is adjusted so as toequalize the coupling coefficients of the respective semiconductorlasers.

[0020] The present invention also provides a manufacturing method forcollectively manufacturing, on a single substrate, an opticalsemiconductor device comprising a plurality of semiconductor laserswhich emit longitudinal single mode laser lights having differentwavelengths due to a distribution feedback operation of a periodicchange of the refractive index in respective semiconductor lasers,wherein the refractive indexes of said diffraction grating embeddedsemiconductor layer are decreased (or increased) so as to cancel thedifference of the coupling coefficients of the respective semiconductorlasers whose coupling coefficients are increased (or decreased) when thediffraction gratings for generating a distribution feedback operationare formed in the same configuration and the refractive indexes of saiddiffraction grating embedded semiconductor layers are fixed at the samevalue.

[0021] In the above manufacturing method for collectively manufacturing,on a single substrate, an optical semiconductor integrated devicecomprises a plurality of semiconductor lasers which emit longitudinalsingle mode laser lights having different wavelengths due to adistributed feedback operation of a periodic change of the refractiveindex in the respective semiconductor lasers, and a plurality of opticalsemiconductor portions integrally formed with said semiconductor lasersfor receiving respective laser lights from said plurality ofsemiconductor lasers, wherein the refractive indexes of said diffractiongrating embedded semiconductor layers are decreased (or increased) so asto cancel the difference of the coupling coefficients of the respectivesemiconductor lasers whose coupling coefficients are increased (ordecreased) when the diffraction gratings for generating a distributedfeedback operation are formed in the same configuration and therefractive indexes of the diffraction grating embedded semiconductorlayers are fixed at the same value.

[0022] In the above manufacturing method, said optical semiconductorintegrated device comprises longitudinal single mode oscillatingsemiconductor lasers and optical modulators.

[0023] In the above manufacturing method, the band-gap wavelengths ofsaid diffraction grating embedded semiconductor layers are made shorter(or longer) so as to cancel the difference of the coupling coefficientsof the respective semiconductor lasers whose coupling coefficients areincreased (or decreased) when the diffraction gratings for generating adistribution feedback operation are formed in the same configuration andthe band-gap wavelength of said diffraction grating embeddedsemiconductor layers are fixed at the same value.

[0024] In the above manufacturing method, said diffraction gratingembedded semiconductor layer is made of InGaAsP, and the change of therefractive index of said InGaAsP layer is executed by changing thecompositional ratio of In and Ga in Group III.

[0025] In the above manufacturing method, said diffraction gratingembedded semiconductor layer is made of InGaAsP, and the change of theband-gap wavelength is executed by changing the compositional ratio ofAs and P in Group V.

[0026] In the above manufacturing method, the method for changing therefractive index or the band-gap wavelength of said diffraction gratingembedded semiconductor layer is a selective metal organic vapor phasegrowth method.

[0027] In the above manufacturing method, the method for changing therefractive index or the band-gap wavelength of said diffraction gratingembedded semiconductor layer is provided by adjusting a flowing ratio ofgroup V group materials in an atmospheric pressure double-fluid layertype metal organic vapor phase epitaxy method.

[0028] An optical communication module is provided by assembly of theabove described semiconductor device or the semiconductor device made bythe above manufacturing method with a waveguide device for guiding anoutput light from said optical semiconductor device to the outside, amechanism for inputting the output light from said semiconductor deviceto the waveguide device, and an electrical interface for driving saidsemiconductor device.

[0029] An optical communication apparatus is provided by assembly of theabove optical semiconductor device or the above semiconductor devicemanufactured by the above-described manufacturing method with an opticaltransmission device loaded with said optical communication module and areceiving device for receiving the output light from said lighttransmission device.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030]FIG. 1 is a cross-sectional view of a semiconductor crystal forexplaining the action of the present invention.

[0031]FIG. 2 is a cross-sectional view of a semiconductor crystal growthfor explaining the action of the present invention.

[0032]FIG. 3 is a characteristic diagram showing the relationshipbetween the mask width and the photoluminescence wavelength forexplaining the action of the present invention.

[0033]FIG. 4 is a characteristic diagram showing the relationshipbetween the mask width and the photoluminescence wavelength forexplaining the action of the present invention.

[0034]FIG. 5 is a characteristic diagram showing the results oftheoretical calculation related to the relationship between thediffraction grating height and the coupling coefficient.

[0035]FIG. 6 is a device characteristic diagram showing the relationshipbetween the oscillating wavelength and the coupling coefficient forexplaining the action of the present invention.

[0036]FIG. 7 is a diagram showing the crystal growth characteristics forexplaining the action of the present invention.

[0037]FIG. 8 is a diagram showing the crystal growth characteristics forexplaining the action of the present invention.

[0038]FIG. 9 is a diagram showing the device characteristics forexplaining the action of the present invention.

[0039]FIG. 10 is a diagram showing the device characteristics forexplaining the action of the present invention.

[0040]FIG. 11 is a diagram showing the device characteristics forexplaining the action of the present invention.

[0041]FIG. 12 is a characteristic diagram showing the results oftheoretical calculations as a parameter of the diffraction gratingheight for explaining the action of the present invention.

[0042]FIG. 13 is a cross-sectional view of a crystal growing apparatusfor explaining the action of the present invention.

[0043]FIG. 14 is a characteristic diagram showing the crystal growthcharacteristics for explaining the action of the present invention.

[0044]FIGS. 15A and 15B are perspective views showing manufacturingprocesses according to one embodiment of the present invention.

[0045]FIG. 16 is a plan view showing a manufacturing process accordingto one embodiment of the present invention.

[0046]FIG. 17 is a diagram showing the theoretical characteristics ofone embodiment of the present invention.

[0047]FIG. 18 is a plan view showing the substrate surface forexplaining one embodiment of the present invention.

[0048]FIG. 19 is a characteristic diagram explaining one embodiment ofthe present invention.

[0049]FIG. 20 is a plan view explaining one embodiment of the presentinvention.

[0050]FIG. 21 is a perspective view explaining one embodiment of thepresent invention.

[0051]FIG. 22 is a perspective view explaining one embodiment of thepresent invention.

[0052]FIG. 23 is a cross-sectional view along the A-A′ line in FIG. 22.

[0053]FIGS. 24A to 24C are cross-sectional views along the C-C′ line inFIG. 22 for explaining the manufacturing processes according to oneembodiment of the present invention.

[0054]FIG. 25 is a perspective view explaining one embodiment of thepresent invention.

[0055]FIG. 26 is a characteristic diagram explaining one embodiment ofthe present invention.

[0056]FIG. 27 is a characteristic diagram explaining one embodiment ofthe present invention.

[0057]FIG. 28 is a characteristic diagram explaining one embodiment ofthe present invention.

[0058]FIG. 29 is a plan view of a substrate surface explaining oneembodiment of the present invention.

[0059]FIG. 30 is a plan view of the substrate surface explaining oneembodiment of the present invention.

[0060]FIG. 31 is a plan view of the substrate surface explaining oneembodiment of the present invention.

[0061]FIG. 32 is a perspective view explaining one embodiment of thepresent invention.

[0062]FIGS. 33A to 33E are cross-sectional views explaining themanufacturing processes according to one embodiment of the presentinvention.

[0063]FIG. 34 is a perspective view showing the structure of oneembodiment of the present invention.

[0064]FIG. 35 is a characteristic diagram explaining one embodiment ofthe present invention.

[0065]FIG. 36 is a characteristic diagram explaining one embodiment ofthe present invention.

[0066]FIG. 37 is a plan view showing the substrate surface explainingone embodiment of the present invention.

[0067]FIG. 38 is a design characteristic diagram explaining oneembodiment of the present invention.

[0068]FIG. 39 is a characteristic diagram explaining one embodiment ofthe present invention.

[0069]FIG. 40 is a design characteristic diagram explaining oneembodiment of the present invention.

[0070]FIG. 41 is a perspective view explaining one embodiment of thepresent invention.

[0071]FIGS. 42A to 42C are cross-sectional views along the C-C′ line inFIG. 41 for explaining one embodiment of the present invention.

[0072]FIG. 43 is a perspective view explaining one embodiment of thepresent invention.

[0073]FIG. 44 is a diagram showing the structure of an opticalcommunication module for explaining an application example of thepresent invention.

[0074]FIG. 45 is a diagram showing an example of the opticalcommunication system for explaining an application example of thepresent invention.

[0075]FIG. 46 is a perspective view showing an example of the collectiveformation of a conventional different wavelength element.

DETAILED DESCRIPTION OF THE INVENTION

[0076] Hereinafter, an embodiment of the present invention is describedwith reference to the attached drawings. The inventors of the presentinvention have examined how the mask width and the coupling coefficientκ affect the properties of the present type of optical semiconductordevice. A series of examinations has revealed an important fact in therelationship between the mask width or the coupling coefficient and theoscillating frequency of the DFB laser. That is, a technique was foundby the present inventors to maintain the coupling coefficient constanteven if the oscillating frequency changes with each DFB laser. Inaddition, several measures were also found for improving thecharacteristics of this type of optical semiconductor devices.

[0077]FIG. 1 shows a schematic cross-sectional view of an InGaAsPcrystal grown by a selective MOVPE method. On the (100) InP substrate 1,an InGaAsP layer was selectively grown as a selective growing layer 3 bythe MOVPE method using a pair of stripe-shaped SiO₂ masks formed in the[011] direction of a crystal plane separated by a space of 1.5 μm. Asshown in FIG. 1, the crystal grows into a trapezoidal form, and the sidefacet and the top surface of the trapezoid are formed by the (111) Bplane and the (100) plane, respectively. When the mask width Wm isincreased, the growth rate (of the film thickness) is increased by thematerial supply from the masked region and it is known that the Incontent in the crystal composition increases. Utilization of thisphenomenon makes it possible to produce, in one substrate surface, aplurality of layers respectively having different band-gap wavelengthsonly by changing mask width.

[0078]FIG. 2 shows a cross-sectional view of the DFB laser in thewaveguide axis direction. The typical DFB laser structure is formed byfirst forming a diffraction grating 100 having a constant height d_(GTG)on an InP substrate 101, and laminated layers on the diffraction gratingcomprising an InGaAsP guide layer 104, a multi-quantum well active layer105, and an InP clad layer 106. In the case of forming the 1.55 μm bandDFB laser, a composition having a band-gap wavelength of 1.0 to 1.3 μmis frequently used as that of the InGaAsP guide layer.

[0079]FIG. 3 shows a relationship between the photoluminescencewavelength and the mask width of the InGaAsP (Q1.13 means InGaAsP whichband gap wavelength is 1.13 μm and Q1.20 means InGaAsP which bandgapwavelength is 1.20 μm) layers grown by the selective MOVPE with thegrowth condition of lattice matching to InP at a mask width of 15 μm.When the band-gap wavelength is 1.13 μm (Q is 1.13), and when the maskwidth is increased to 50 μm, the photoluminescence wavelength can beextended to 1.20 μm.

[0080] This extension of wavelength is due to the strain caused by theincrease of the In content in the selectively grown layer induced by theincrease of the mask width.

[0081]FIG. 4 shows an example of the relationship between thephotoluminescence wavelength and the mask width when the MQW structurecomposed of InGaAsP/InGaAsP is grown by the selective MOVPE. In thiscase, the band-gap wavelength can cover a range of wavelengths from 1.52μm to 1.62 μm when the mask width is increased from 15 μm to 50 μm.Moreover, the wavelength range of 1.45 μm to 1.55 μm can be covered whenthe mask width is increased from 5 μm to 21 μm. This wavelength range of1.45 μm to 1.55 μm can be used as an operating wavelength range for theelectro-absorption (EA) modulator to be integrated in combination withthe above-described DFB laser having an oscillating wavelength range of1.52 μm to 1.62 μm.

[0082] The issue is the wavelength dependency of the couplingcoefficient κ when manufacturing the DFB lasers. At the time ofmanufacturing a plurality of different DFB lasers having a wavelengthrange of 1.52 μm to 1.62 μm by changing the mask width within a range of15 μm to 50 μm, when the guide layer is formed using InGaAsP having aband-gap wavelength of 1.13 μm with a mask width of 15 μm on thediffraction grating, the photoluminescence wavelength of the guide layerchanges from 1.13 μm to 1.20 μm. The change of the photoluminescencewavelength means a change in the band-gap wavelength induced by thechange of the refractive index of the guide layer, which implies thatthe coupling coefficient κ changes due to the periodic change of therefractive index produced by the diffraction grating.

[0083]FIG. 5 shows the results of the calculation between the heightd_(GTG) of the diffraction grating, which has a proportionalrelationship with the change value of the periodic change of therefractive index, and the coupling coefficient using the composition ofthe guide layer as a parameter. When the band-gap wavelength changeswith the change of the composition of the guide layer from 1.13 μm to1.20 μm, it is anticipated that the coupling coefficient κ changes from37.5 to 53 cm⁻¹.

[0084] An experiment was performed, obtaining the results shown in FIG.6, for measuring the relationship between the coupling coefficient κ andthe DFB oscillation wavelength of DFB lasers collectively formed in asingle wafer having a wavelength range of 1.52 μm to 1.62 μm. In theabove experiment, two different diffraction grating heights of 30 nm and35 nm were tested. As a result, it was found that the couplingcoefficient κ is changed from 30 to 40 cm⁻¹ when the diffractiongratings height is 30 nm, and from 38 to 50 cm⁻¹ when the height is 35nm.

[0085] The above results show that the variations in the couplingcoefficient κ for both diffraction grating heights are smaller thanthose expected from FIG. 5. This is due to the fact that the calculationin FIG. 5 did not take into account the increase of the lightconfinement coefficient in the laser active layer by the increase of thefilm thickness due to the increase of the mask width.

[0086] Below, a technique is described with reference to FIGS. 7 and 8for collective formation of laser active layers having differentband-gaps in one substrate surface by the selective MOVPE growth withoutchanging the mask width.

[0087] In vertical MOVPE reactors, when the amount of the carrier gas isincreased to move than the optimum amount for growing a film thicknesshomogeneous, the growth rate can be increased from the center to theouter periphery of the substrate (the homogeneity is degraded).Similarly, in horizontal MOVPE reactors, when the flow rate is increasedto move higher than the optimum amount for growing the film thicknesshomogeneous, a distribution can be provided, in which the growth rate isincreased from upstream to downstream in the direction of the gas flow.In contrast, if the MOVPE growth is carried out at a gas amount lessthan the optimum amount to for a film with a homogeneous thickness, thereverse film thickness distribution can be obtained.

[0088]FIG. 7 shows an example of the film thickness distribution whenthe flow rate is increased to 1.5 times higher than the optimum flowrate. As shown in FIG. 7, the growth rate distribution is represented asa function of the normalized growth rate, wherein the growth rate at thecenter of a 2 inch InP substrate is set at 1, against the distance fromthe center of the substrate. From the figure, the growth rate at theoutermost periphery of the substrate is about 1.4 times larger than thatat the center of the substrate.

[0089] The result of the photoluminescence (PL) wavelength distributionin the substrate surface of the InGaAsP/InGaAsP multi-quantum wellstructure formed by selective growth using the above film thicknessdistribution is shown in FIG. 8. The multi-quantum well structure is thesame as that shown in FIG. 2, and FIG. 8 shows two examples for whenrespective mask widths are 6 μm to 16 μm. When the mask width Wm=6 μm,the PL wavelength in the substrate surface distributed within awavelength range of 1.46 μm (1460 nm) to 1.51 μm (1510 nm), and when themask width Wm=16 μm, the PL wavelength distributed within a wavelengthrange of 1.53 μm (1530 nm) to 1.58 μm (1580 nm).

[0090] Accordingly, utilization of the structure having the abovewavelength distribution makes it possible to execute the collectiveformation of the DFB lasers having different oscillation wavelengths onone substrate surface. It is possible to produce DFB lasers having awavelength range of 1.53 μm to 1.58 μm by setting the mask width at 16μm. The wavelength range of 1.46 μm to 1.51 μm can be covered by settingthe mask width at 6 μm, and this wavelength range can be used as theoperating wavelength range of the electro-absorption (EA) modulator forDFB lasers having an oscillating wavelength range of 1.53 μm to 1.58 μm.

[0091] An issue is the wavelength dependency of the couplingcoefficients κ within the above oscillating wavelength range. Since thethickness of the MQW active layers in the different wavelengthoscillating DFB lasers within the wavelength range of 1.53 μm to 1.58 μmreaches 1.4 times at maximum compared with the minimum, the opticalfield of the diffraction grating portion becomes weak because the lightconfinement coefficient in the MQW active layer of the long wavelengthlasers having the higher layer thickness becomes large.

[0092] The change of the long distance transmission characteristics dueto the change of the coupling coefficients κ is originated by thewavelength variation (wavelength chirp) induced by the residualreflection light from the end facet at the time of the modulationoperation.

[0093]FIG. 10 shows the relationship between the coupling coefficients κand the power penalty after a 2.5 Gb/s modulated transmission over 600km (indicated by broken line ). The measurement was made using a deviceproduced by monolithic integration of a 400 μm long uniform diffractiongrating-type DFB laser with a 200 μm long EA modulator. The powerpenalty after transmission over 600 km decreases with an increase of thecoupling coefficients κ, and a power penalty less than 1 dB is obtainedwhen the coupling coefficients κ>37 cm⁻¹. In contrast, the relationship(the real line ◯) between the coupling coefficient κ and thelongitudinal single mode oscillation yield shows that the yield isreduced by the larger effect of the spatial hole burning as the couplingcoefficient κ increases. Therefore, it is very important to control thecoupling coefficient κ to within a certain range.

[0094]FIG. 11 shows a similar relationship between the power penalty andthe product κL of the coupling coefficient κ and the DFB laseroscillator length L and the product κL must be more than 1.5 in order toobtain a power penalty less than 1 dB.

[0095] In the case of collective formation of the different wavelengthDFB laser, the elements integrated in the multiple wavelength lasernecessarily involve the change of the coupling coefficient κ, since eachlaser active layer structure (composition or film thickness) differsfrom each other in order to provide elements having differentwavelengths.

[0096] However, it becomes possible to control the change of thecoupling coefficient κ, since the coupling coefficient κ can becontrolled not only by the change of the height of the diffractiongrating but also by the composition of the guide layer (band-gapwavelength) on the diffraction grating, as shown in FIG. 5. That is, thechange of the coupling coefficient κ can be cancelled by controlling theband-gap wavelength of the guide layer on the diffraction gratingindependent from the MQW structure which determines the band-gapwavelength of the laser active layer.

[0097]FIG. 12 shows the results of the calculation regarding therelationship between the band-gap wavelength and the couplingcoefficient κ when changing the height of the diffraction grating asparameters. In this case, the calculation is carried out based on theMQW structure of the DFB laser with an oscillating wavelength of 1.52μm, whose coupling coefficient κ dependent on the oscillating wavelengthis shown in FIG. 9. As shown in FIG. 12, the coupling coefficient κincreases into two times from 37.5 cm⁻¹ to 75 cm⁻¹ when the band-gapwavelength is varied from 1.13 μm to 1.30 μm.

[0098] Accordingly, when collectively manufacturing a DFB laser having awavelength from 1.52 μm to 1.58 μm (the coupling coefficient κ isreduced to 50% for elements having wavelengths from 1.52 μm to 1.58 μm),it is possible to maintain the coupling coefficient at a constant valueby changing the band-gap wavelength of the guide layer from 1.13 μm to1.30 μm.

[0099] Next, a few methods are provided for controlling the band-gapwavelength of the guide layer independently of the MQW. The first methodis to form the guide layer on the diffraction grating and the MQW activelayer by respective selective MOVPEs. It is possible to change theband-gap wavelength (photoluminescence wavelength) from 1.12 μm to 1.30μm within a mask width of 2 to 50 μm when the InGaAsP layer having theband-gap wavelength of 1.20 μm (Q1.20) is grown by the selective MOVPEat the mask width of 15 μm under the lattice matching condition.

[0100] The second method is to grow the guide layer under intentionallynon-homogeneous growing condition using an atmospheric pressure doublefluid layer flow-type MOVPE. Although the atmospheric pressure doublefluid layer flow-type MOVPE is disclosed in detail in The Journal ofCrystal Growth, 145, p. 622, 1994, the principle is briefly describedbelow with reference to FIG. 13.

[0101] The reaction tube 501 of the atmospheric pressure double fluidlayer flowing-type MOVPE reactor has a material inlet, which is dividedinto two upper and lower layer-shaped inlets with respect to thesemiconductor substrate 503 supported by the substrate holder 502, asshown in FIG. 13. The reaction tube 501 is constituted such that arsine(AsH₃) and phosphine (PH₃), which belongs to Group V, are supplied fromthe upper layer-shaped inlet located near the substrate and thematerials, which belongs to Group III such as trimethylgallium (TMGa),or trimethylindium (TMIn), are supplied from the lower layer-shapedinlet located far from the substrate.

[0102] Generally speaking, the reason it is difficult to carry out thehomogeneous growth of the InGaAsP by the MOVPE growth is that theexisting ratio of As and P atoms differs on the substrate surface due tothe large difference in the decomposition rate constants of AsH₃ and PH₃(the decomposition rate of AsH₃ is about ten times larger than that ofPH₃ t a growth temperature of 600 to 650° C.). Thus, this MOVPE growthis characterized in that PH₃ having a small decomposition rate constant,is supplied from the upper layer-shaped inlet located close to thesubstrate, and AsH₃ is supplied from both upper and lower inlets suchthat the homogeneous growth of the InGaAsP is attained by conforming thenumbers of As atoms with P atoms reaching the substrate surface byadjusting the AsH₃ flow rate of the upper inlet and the lower inlet.

[0103] Thus, when the crystal growth of the guide layer is carried outunder a condition, which are shifted from that for homogeneous growth bychanging the flow-rates from the upper and lower layer-shaped inlets, itis possible to change the photoluminescence wavelength of the guidelayer along the flowing direction of the material gas. FIG. 14 shows anexample. In FIG. 14, the abscissa shows a gas flow ratio (=the partialflow rate of AsH₃ from the upper inlet divided by the total flow rate ofAsH₃ from both upper and lower inlets) and the vertical axis showsdifferences of the photoluminescene wavelengths of the grown crystal ofQ1.25 (InGaAsP having a composition corresponding to the band-gapwavelength of 1.25 μm) at the material gas inlet side and the materialgas outlet side. As shown in FIG. 14, maximum homogeneous growth isobtained at a flow ratio of 60%, and when the flow ratio is lower than60%, the wavelength of the crystal at the material gas inlet side shiftsto a longer wavelength and when the flow ratio is higher than 60%, thewavelength of the crystal at the gas outlet side shifts to a longerwavelength.

[0104] The above-described experimental results indicates that it ispossible to provide a maximum band-gap wavelength difference of 100 nm.Accordingly, the variation of the coupling coefficient can be cancelledby collectively forming the different wavelength DFB laser such that theoscillating wavelengths of the crystal grown on the substrate changealong the direction of the gas flow and by growing the guide layer onthe diffraction grating by the above-described non-homogeneous conditionsuch that the band-gap wavelength changes due the direction of the gasflow.

[0105] Hereinafter, the manufacturing method of the semiconductor deviceof the present invention is explained in detail with reference to theattached drawings.

[0106] First, a method for collective manufacturing the different waveEA modulator integrated-type DFB laser according to the first embodimentof the present invention is described. As shown in FIG. 15, aftercoating the electron beam sensitive positive resist 102 on the (100)n-InP substrate 101, a pattern, as shown in FIG. 15A, is formed byelectron-beam exposure.

[0107] Subsequently, a diffraction grating, as shown in FIG. 15B, isformed by etching the n-InP substrate 101 with an HBr:H₂O-type etchingsolution using the resist 102 as the mask.

[0108] The diffraction grating is formed as a pattern, in which adiffraction grating formation region with a length of 800 μm in the[011] direction and a non-diffraction grating formation region with alength of 400 μm are repeatedly formed in the [01-1] direction.

[0109] In order to provide an EA modulator integrated DFB laser, thepitch Λ of the diffraction grating is set within a range of 235.05 to244.30 nm which can yields oscillating wavelengths ranging from 1.52 to1.58 μm.

[0110] Subsequently, a pair of stripe masks 103 g made of SiO₂ forforming a guide layer on the diffraction grating by the selective MOPVEgrowth is obtained by patterning as shown in FIG. 18. The relationshipbetween the width W_(G) of the SiO₂ mask for the selective MOPVE growthand the DFB laser oscillating wavelength is shown in FIG. 19. In thecollective formation of the DFB laser having a wavelength range of 1.52μm to 1.58 μm, the width W_(G) is characterized in being set larger asthe DFB oscillating wavelength increases to a longer wavelength, whereinthe coupling coefficient κ becomes smaller, as shown in FIG. 9. Then-InGaAsP guide layer 104 (band-gap wavelength is 1.20 μm, the thicknessis 150 nm, n=1×10¹⁸ cm⁻³) is grown by the MOVPE growth in the region ofW_(G)=15 μm under a lattice matching condition.

[0111] It has been confirmed from the microscopic photoluminescencemeasurement of the selectively grown guide layer that the mask widthdependence of the band-gap wavelength can be reproduced as shown in FIG.3.

[0112] Subsequently, a pair of SiO₂ masks used for the selective MOPVEgrowth is re-patterned so as to be synchronous with the diffractiongrating 100. Here, as shown in FIG. 20, the mask width is set at W_(LD)in the region wherein the diffraction grating is formed, and the maskwidth is set at W_(MOD) in the region wherein no diffraction grating isformed. Although not shown in FIG. 20, the n-Q1.20 guide layer is formedin the aperture region where no diffraction grating is formed, asexplained in FIGS. 18 and 19.

[0113]FIG. 21 illustrates the SiO₂ mask pattern for one element (then-Q1.20 guide layer is not shown in this figure). That is, the regionwhere the mask width is set at W_(LD) (400 μm in length) is used forforming the DFB laser, and the region where the mask width is set atW_(MOD) (200 μm in length) is used for forming the EA modulator. Thespace between one pair of SiO₂ stripe masks is fixed at 1.5 μm. TheW_(LD) is fixed at 16 μm and the W_(MOD) is set at 6 μm.

[0114] In FIGS. 17 and 19, the pitch Λ and the width W_(G) of thediffraction grating are shown to have a continuous relationship with theoscillating frequencies. However, in this embodiment, since the range ofthe oscillating wavelengths 1.52 μm to 1.58 μm are divided into 75wavelengths by an interval of 100 GHz (approximately 0.8 nm), the actuallines in FIGS. 17 and 19 are actually composed of 75 discontinuous dots.

[0115] Subsequently, the MQW laser active layer and the modulatorabsorption layer are grown by the selective MOVPE growth. The crystalgrowth by the selective MOVPE growth is carried out under conditions soas to provide a film having the thickness distribution as shown in FIG.7.

[0116] On the n-InGaAsP guide layer 104, already described in FIG. 18, astrained MQW layer 105 (8 cycles of well layers of InGaAsP (+0.60%compressively strained, 10 nm thick) and barrier layers of InGaAsP (theband-gap wavelength is 1.20 μm, 9 nm in thickness)) and a p-InP layer106 (150 nm in thickness, p=5×10¹⁷ cm⁻³) are formed. Thephotoluminescent wavelengths in the W_(LD) layer (=16 μm) and theW_(MOD) layer (=6 μm) distribute respectively within a range of 1.52 to1.58 μm and a range of 1.45 to 1.51 μm.

[0117] As shown in FIG. 23, which is a cross-sectional view along theA-A′ line shown in FIG. 22, the diffraction grating is present in theDFB laser region. The band-gap wavelength of the Q1.20 guide layer 104on the diffraction grating changes from 1.13 μm to 1.30 μm correspondingto the changing design oscillating wavelengths. Consequently, it isexpected that the coupling coefficient κ can be maintained at κ=37.5cm⁻¹ irrespective of the change of the oscillating frequency.

[0118] As shown in FIG. 24, which is a cross-sectional view along theC-C′ line shown in FIG. 22, both side portions of the SiO₂ layer locatedat both sides of the selective growth layer are removed (FIG. 24B),after forming a p-InP clad layer 107 (1.5 μm in thickness and p=1×10¹⁸cm⁻³) and a P⁺-InGaAs cap layer 108 (0.25 μm in thickness, p=6× 10¹⁸cm⁻³) by the selective MOVPE growth as shown in FIG. 24C, electricalseparation is performed by removing the p⁺-InGaAs cap layer 108 with awidth of 30 μm between the DFB laser region and the EA modulator region.As shown in FIG. 25, a p-type electrode 110 is formed by patterningusing the SiO₂ film 109 as the interlayer insulating film, and an n-typeelectrode 111 is formed after back grinding is carried out until thethickness of the n-InP substrate is reduced to 120 μm. A total length of600 μm of the thus formed substrate including the DFB laser portion of400 μm in length and the EA modulator portion of 200 μm in length is cutinto elements. Each element is subjected to the evaluation of the devicecharacteristics after coating a high reflection film with a reflectanceof 95% at the end facet of the DFB laser portion and after coating ananti-reflection film with less than 0.1% reflectance at the end facet ofthe modulator.

[0119] First, the oscillating wavelength characteristics of thisdifferent wavelength collectively formed EA modulator integrated DFBlaser are examined. DFB laser oscillating wavelengths of the 75 thusproduced elements are plotted in FIG. 26 in sequence. It is confirmedthat the wavelengths of these elements from channel 1 to channel 75 areshifted from one to the next channel by 0.8 nm as intended and multiplewavelength laser elements covering the wavelength range of 1520 to 1580nm (1.52 to 1.58 μm) are provided on one substrate surface.

[0120] Next, the coupling coefficients κ for some elements havingdifferent wavelengths are evaluated. Consequently, it has beenconfirmed, as shown in FIG. 27, that the coupling coefficient ismaintained approximately at a designed value of 37.5 cm⁻¹ irrespectiveof the change of the oscillating wavelength. This result is obtained byutilizing the effect of the present invention that the band-gapwavelengths of the InGaAsP guide layers for respective elements on thediffraction grating are changed corresponding to the oscillatingwavelengths of the elements.

[0121] As a result, as shown in FIG. 28, the transmission yield,obtained by use of a standard that the power penalty after 2.5 Gb/stransmission over 600 km is less than 1 dB, is more than 95% in allwavelength ranges, while the single longitudinal mode oscillation yieldis maintained to more than 60% irrespective of the oscillationwavelength.

[0122] Next, a collective manufacturing method for a differentwavelength DFB laser according to the second embodiment is describedbelow. Similar to the first embodiment, the diffraction grating isformed by electron beam exposure and etching. As shown in FIG. 29, thediffraction grating is formed on the entire region of the [01-1]direction, and a repeating pattern is formed t a pitch of 300 μm in the[01-1] direction. In order to provide a different wavelength DFB laser,the diffraction grating pitch Λ is controlled from 235.05 to 244.30 nmin order to control the DFB oscillating wavelengths to within a range of1.52 μm to 1.58 μm, corresponding to the DFB oscillating wavelength. Inthis embodiment, the etching amount shown in FIG. 15B is controlled soas to obtain a grating height of 30 nm.

[0123] Subsequently, a plurality of SiO₂ stripe masks 203 g are formedby the patterning process for growing the guide layer on the diffractiongrating 200 by the MOVPE growth. The mask width is represented by W_(G).The relationship shown in FIG. 19, between the mask width W_(G) forforming the guide layer by the MOVPE growth and the oscillatingfrequencies of the DFB lasers is used for forming the SiO₂ masks.

[0124] Subsequently, using these SiO₂ stripe masks, an n-InGaAsP guidelayer 204 (the band-gap wavelength is 1.20 μm, the film thickness 150nm, n=1×10¹⁸ cm⁻³) is formed between the mask region of W_(G)=15 μm bythe selective MOVPE growth method under a lattice matching condition.The microscopic photoluminescence measurement of the band-gap wavelengthof the selective grown guide layer clearly shows that the mask widthdependence on the band-gap wavelengths has been reproduced as shown inFIG. 3.

[0125] Subsequently, as shown in FIG. 31, a pair of SiO₂ stripe masksfor growing the MQW active layer by the selective MOVPE growth method isformed by re-patterning so as to synchronize with the diffractiongrating. Although not shown in FIG. 31, the n-Q1.20 guide layer ispresent in the aperture region of the SiO₂ stripes.

[0126]FIG. 32 illustrates a SiO₂ mask pattern for one element (then-Q1.20 guide layer is not shown in this figure). The interval of thepair of stripe masks is fixed at 1.5 μm. Although the pitches Λ andW_(G) of the diffraction grating are shown continuously as lines inFIGS. 17 and 19, the pitches of the present embodiment are cut into 75wavelengths by dividing the wavelength range of 1.52 μm to 1.58 μm into75 steps with the interval of 100 GHz (approximately 0.8 nm), so that itis noted that the lines in FIGS. 17 and 19 are composed of 75 steps ofdiscontinuous values.

[0127] Next, the selective MOVPE growth process is described withreference to FIG. 33, which is a cross-sectional view of the D-D′ lineshown in FIG. 32. The MOVPE growth is conducted under a condition suchthat the film is grown with a thickness distribution as shown in FIG. 7.

[0128] On the n-InGaAsP guide layer 204, explained in FIGS. 19 and 30,the strained MQW layer (five cycles of the well layer: InGaAsP (+0.60%strain, 10 nm in thickness) and the barrier layer: InGaAsP 205 (band-gapwavelength 1.20 μm, 9 μm in thickness)), and the p-InP layer 206 (150 nmin thickness, p=5×10¹⁷ cm⁻³), (these values are obtained when W_(LD)=15μm) are grown by the selective MOVPE growth.

[0129] Subsequently, after forming SiO₂ mask 207 as shown in FIG. 33C onthe above-described layer formed by the selective MOVPE method, a p-InPblock layer 208, an n-InP block layer 209, and a p-InP layer 210 areselectively grown as shown in FIG. 33D. Finally, after removing the SiO₂mask, a p-InP clad layer 211 and a p⁺-InGaAs cap layer 212 are grown bythe selective MOVPE growth method.

[0130] The processes shown in FIGS. 33B to 33E are disclosed in detailin a paper published in IEEE Journal of Quantum Electronins. Vol. 35,No. 3, pp. 368-367”.

[0131] Subsequently, as shown in FIG. 34, a p electrode 214 is formed bypatterning using the SiO₂ film as an interlayer insulating film, andafter back-grinding of the n-InP substrate 201 until obtaining athickness of 100 μm, an n-type electrode 215 is formed on the backsurface. After formation of the above structure, an element is obtainedby cutting the substrate 210 into a length of 400 μm, andcharacteristics of the element are evaluated by coating a highreflectance film having a 95% reflectance on the one end facet of theDFB laser and coating a non-reflectance film having a reflectance of0.1% on the other end facet.

[0132] First, the results of the evaluation of the oscillatingwavelength characteristics of the different wavelength collectivelyformed DFB laser are described. The oscillating wavelengths of the DFBlaser composed of 75 elements are shown in FIG. 26. The wavelengths ofrespective elements from channel 1 to channel 75 are shifted stepwise tolonger wavelengths by a slope of 0.8 nm/channel, and it is confirmedthat different wavelength elements are provided on one substratecovering a range of 1520 to 1580 nm (1.52 to 1.58 μm). The couplingcoefficient κ of these laser elements are examined.

[0133] It was confirmed as shown in FIG. 35 that the couplingcoefficients κ for these laser elements are maintained at anapproximately constant value of 30.0 cm⁻¹. The stable oscillatingwavelength dependency of the coupling coefficients κ for the presentlaser elements is obtained as a result of the present invention in whichthe band-gap wavelengths of the InGaAsP guide layer are variedcorresponding to the oscillating wavelength in contrast to theconventional coupling coefficients κ with the oscillating wavelength.

[0134] Consequently, a most preferable results were obtained in that thehighly homogeneous characteristics such as the laser oscillatingthreshold value 5.0±0.25 mA, the efficiency 0.35±0.05 W/A, and themaximum light output 160±3 mW are resulted, and the longitudinal singlemode yield is more than 65% irrespective of the oscillating wavelength.

[0135] Next, a collective manufacturing method according to the thirdembodiment of the present invention is described for manufacturing adifferent wavelength EA modulater integrated DFB laser differing fromthe first embodiment.

[0136] As shown in FIG. 15, an electron beam sensitive positive resist102 is coated on the (100) n-InP substrate 101, and the pattern shown inFIG. 15A is formed. Subsequently, the diffraction grating shown in FIG.15A is formed by etching the n-InP substrate 101 using an etchingsolution in the system of HBr:H₂O using of the resist 102 as the mask.The diffraction grating is formed as a pattern in which a diffractiongrating formation region with a length of 800 μm in the [011] directionand a non-diffraction grating formation region with a length of 400 μmare repeatedly formed in the [01-1] direction. In order to provide an EAmodulator integrated DFB laser, the pitch Λ of the diffraction gratingis set within a range of 235.05 to 244.30 nm which can yield oscillatingwavelengths ranging from 1.52 to 1.58 μm. The arrangement of thisdifferent pitch in the diffraction grating is explained with referenceto FIGS. 37 and 38.

[0137] The position coordinate of the 2 inch InP substrate is defined inFIG. 37. In the figure, the left end of the substrate is defined as X=0mm, the right end of the substrate is defined as X=50 mm, and the centerof the substrate is defined as X=25 mm. Based on this definition, thepitches of the diffraction grating on the substrate areone-dimensionally disposed such that 235.05 nm at X=5 mm and 244.30 nmat 45 mm are obtained. Subsequently, this substrate with the diffractiongrating is introduced into the double fluid layer-type MOVPE growthapparatus for growing an n-InGaAsP guide layer (the band-gap wavelengthis 1.25 μm, the thickness is 150 nm, n=1×10¹⁸ cm⁻³) on the entiresubstrate surface. In the growing process, the substrate end at X=0 isdisposed upstream and the substrate end at X=50 mm is disposeddownstream for the flow of the material gas. The AsH₃ partial flow ratiois set at 85%.

[0138]FIG. 39 shows the substrate position dependence of thephotoluminescence wavelength formed by the above-described twolayer-type MOVPE growth. The wavelength becomes longer linearly as theposition shifts from 1.17 μm at X=0 mm to 1.25 μm at X=50 mm.Subsequently, a pair of SiO₂ stripe masks for the selective MOVPE growthis re-patterned as shown in FIG. 20 so as to synchronize with thediffraction grating 100. The mask width is set at W_(LD) in the regionwhere the diffraction grating is formed, and the mask width is set atW_(MOD) in the region where the diffraction grating is not formed.

[0139] Although not shown in FIG. 20, the n-Q1.25 guide layer is formedon the entire substrate surface. FIG. 21 shows the SiO₂ stripe maskpattern for one element (the n-Q1.25 guide layer is also not shown inFIG. 21). That is, the area (400 μm in length) where the mask width isformed at W_(LD) is the region for forming the DFB laser, and the area(200 μm length) where the mask width is formed at W_(MOD) is the regionfor forming the EA modulator.

[0140] The width of a pair of stripe masks is fixed at 1.5 μm. Thewidths of W_(LD) and W_(MOD) are varied corresponding to the DFBoscillating wavelength as shown in FIG. 40. Although the relationshipbetween the pitches Λ including W_(LD) and W_(MOD) of the diffractiongrating and the DFB oscillating wavelength are depicted as two straightlines, it is noted that these lines consist of 75 discontinuous valuescorresponding to the design values obtained by dividing the DFBoscillating wavelength range of 1.52 to 1.58 μm by 100 GHz(approximately 0.8 nm intervals).

[0141] Subsequently, on the substrate 104 on which the SiO₂ pattern isformed, a MQW laser active layer and a modulator absorption layer areformed using the selective MOVPE growth method. On the n-InGaAsP guidelayer 104, eight cycles of the strained MQW layer 105 (eight cycles ofthe well layer: InGaAsP (+0.6% compressively strained, 10 nm inthickness) and the barrier layer of InGaAsP (the band-gap wavelength1.20 μm, 9 nm in thickness)), and p-InP layer 106 (50 nm thick, p=5×10¹⁷ cm⁻³) are formed (all values are obtained when the widthW_(LD)=15 μm). As a result, the photoluminescence wavelengths in therange of 1.52 to 1.58 μm and in the range of 1.45 to 1.51 μm arereproduced for respective widths of W_(LD) and W_(MOD) as shown in FIG.4.

[0142] As shown in FIG. 23, illustrating a cross-sectional view of theA-A′ line in FIG. 41, the diffraction grating exists in the DFB laserregion. The bandgap wavelength of the guide layer 104 changes from 1.17μm to 1.25 μm as designed. Thus, the coupling coefficient κ is expectedto be maintained at a fixed value of 37.5 cm⁻¹.

[0143] As shown in FIG. 42, which is a cross-sectional view along theC-C′ line shown in FIG. 41, both side portions of the SiO₂ layer locatedat both sides of the selective growth layer are removed (FIG. 42B), andafter forming a p-InP clad layer 107 (1.5 μm in thickness and p=1×10¹⁸cm⁻³) and a P⁺-InGaAs cap layer 108 (0.25 μm in thickness, p=6×10¹⁸cm⁻³) by the selective MOPVE growth as shown in FIG. 42C, electricalseparation is performed by removing the p⁺-InGaAs cap layer 108 with awidth of 30 μm between the DFB laser region and the EA modulator region.As shown in FIG. 43, a p-type electrode 110 is formed by patterningusing the SiO₂ film 109 as the interlayer insulating film, and an n-typeelectrode 111 is formed after back grinding is carried out until thethickness of the n-InP substrate is reduced to 120 μm. A total length of600 μm of the thus formed substrate including the DFB laser portion of400 μm in length and the EA modulator portion of 200 μm in length, iscut into elements. Each element is subjected to the evaluation of thedevice characteristics after coating a high reflection film with areflectance of 95% at the end facet of the DFB laser portion and aftercoating an anti-reflection film with less than 0.1% reflectance at theend facet face of the modulator.

[0144] First, the oscillating wavelength characteristics of thisdifferent wavelength collectively formed EA modulator integrated DFBlaser are examined. The DFB laser oscillating wavelengths of the 75 thusproduced elements are plotted in FIG. 26 in sequence. It is confirmedthat the wavelengths of these elements from channel 1 to channel 75 areshifted to longer wavelengths from one to the next channel by 0.8 nm asdesigned and multiple wavelength laser elements covering the wavelengthrange of 1520 to 1580 nm (1.52 to 1.58 μm) are provided on one substratesurface.

[0145] Next, the coupling coefficients κ for some elements havingdifferent wavelengths are evaluated. Consequently, it has been confirmedas shown in FIG. 27 that the coupling coefficient is maintainedapproximately at the designed value of 37.5 cm⁻¹ irrespective of thechange of the oscillating wavelength. This result is obtained byutilizing the effect of the present invention that the band-gapwavelengths of the InGaAsP guide layers for respective elements on thediffraction grating are changed corresponding to the oscillatingwavelengths of the elements.

[0146] As the result, as shown in FIG. 28, the transmission yield,obtained by use of a standard that the power penalty after the 2.5 Gb/stransmission over 600 km is less than 1 dB, is more than 95% in allwavelength ranges, while the single longitudinal mode oscillation yieldis maintained to more than 60% irrespective of the oscillationwavelength.

[0147] In the above embodiment, although examples are described in whichthe diffraction grating, the guide layer, and the active layer areformed in sequence, it is noted that this invention is not limited tothese embodiments and the present invention can be applied to a modifiedstructure in which the guide layer and the diffraction grating can beformed on the active layer.

[0148] As an application example of this invention, a lightcommunication module provided with an optical semiconductor device isdescribed with reference to FIG. 44. This module, loaded with anelectric interface 301 for driving the optical semiconductor device 302,is constituted such that the output light from the optical semiconductordevice 302 is input into the optical fiber 305 through an asphericallens and an optical isolator. This module, constituted as describedabove, makes it possible to effectively execute high speed opticaltransmission. This effect is obtained through the low threshold valueand highly efficient operating characteristics of the opticalsemiconductor device of the present invention.

[0149] As another application example, a WDM optical communicationsystem provided with the optical semiconductor device of the presentinvention is described with reference to FIG. 45. The opticalcommunication apparatus 400 comprises 64 optical communication modules300, a multiplexer 405 for multiplexing multiple lights from these lightemitting elements in the optical communication module 300, and a drivingsystem 402 for driving the light transmission module. A signal lightoutput from the optical communication apparatus 400 is transmitted tothe optical receiving apparatus 401 through the optical fiber 305. Thelight having 64 wavelengths which is received by the optical receivingapparatus is divided into 64 light signals by the optical demultiplexer406 and each divided light signal is input into each of 64 lightreceiving modules 403, driven by the receiving module driving system404. The WDM communication system is realized by the opticalcommunication system of the present invention. This is due to the factthat the optical semiconductor device has quite homogeneouscharacteristics irrespective of the variation of the wavelengths.

[0150] The optical semiconductor device and method for manufacturing thesame according to the present invention is constituted as describedabove. The manufacturing method for collectively manufacturing themultiple wavelength DFB laser or the multiple wavelength EA modulatorintegrated DFB laser is capable of maintaining the coupling coefficientsκ of respective elements by controlling the refractive index (theband-gap wavelength) of the guide layer corresponding to the dielectricmaterial mask width or the location on the substrate. Thereby, itbecomes possible to maintain the coupling coefficient κ or the productκL of the coupling coefficient and the length of the DFB laserresonator, which results in an improvement in the long distancetransmission yield or the longitudinal single mode oscillation yield.

What is claimed is:
 1. An optical semiconductor device comprising aplurality of semiconductor lasers formed on a single substrate, whereineach of said semiconductor lasers emits laser lights having oscillatingwavelengths differing from each other by different cycles of a pluralityof diffraction gratings, wherein a composition of an optical guide layerin contact with one of said diffraction gratings is determined such thatthe coupling coefficient of each of said semiconductor lasers ismaintained at the same value.
 2. An optical semiconductor devicecomprising a plurality of semiconductor lasers formed on a singlesubstrate, wherein each of said plurality of semiconductor lasers emitslongitudinal single mode laser lights having different oscillatingwavelengths due to a distributed feedback operation of a periodic changeof the refractive index in said plurality of semiconductor lasers andwherein said plurality of semiconductor lasers have the same couplingcoefficient by being provided with a diffraction grating embeddedsemiconductor layer each having a refractive index corresponding to theoscillating wavelength.
 3. An optical semiconductor device according toclaim 1 , wherein said semiconductor laser is a distributed feedbacksemiconductor laser.
 4. An optical semiconductor device according toclaim 1 , wherein each of said plurality of diffraction gratings has thesame height.
 5. An optical semiconductor device according to claim 1 ,wherein each of said plurality of semiconductor lasers comprises adiffraction grating embedded semiconductor layer made of InGaAsP havinga band-gap wavelength (energy) corresponding to the oscillatingwavelength thereof.
 6. An optical semiconductor device according toclaim 1 , wherein said substrate is made of InP, said diffractiongratings are formed to the same height from said substrate, and saidoptical guide layer is made of InGaAsP formed on said diffractiongratings.
 7. An optical semiconductor device according to claim 1 ,wherein an optical modulator is monolithically integrated with saidsemiconductor laser.
 8. An optical semiconductor device comprising aplurality of semiconductor lasers provided with a plurality ofdiffraction gratings having cycles different from each other, an InGaAsPguide layer formed on or below said diffraction gratings, amulti-quantum well layer, and an InP clad layer on an InP substrate, andwhich emit laser lights having different wavelengths determined by thecycle of said diffraction gratings, wherein the refractive index of saidguide layer is adjusted so as to equalize the coupling coefficients ofthe respective semiconductor lasers.
 9. A manufacturing method forcollectively manufacturing, on a single substrate, an opticalsemiconductor device comprising a plurality of semiconductor laserswhich emit longitudinal single mode laser lights having differentwavelengths due to a distribution feedback operation of a periodicchange of the refractive indexes in the respective semiconductor lasers,wherein the refractive indexes of diffraction grating embeddedsemiconductor layers are decreased (or increased) so as to cancel thedifference of the coupling coefficients of respective semiconductorlasers whose coupling coefficients are increased (or decreased) when thediffraction gratings for generating a distribution feedback operationare formed in the same configuration and the refractive indexes of saiddiffraction grating embedded semiconductor layers are fixed at the samevalue.
 10. A manufacturing method for collectively manufacturing, on asingle substrate, an optical semiconductor integrated device comprisinga plurality of semiconductor lasers which emit longitudinal single modelaser lights having different wavelengths due to a distributed feedbackoperation of a periodic change of the refractive indexes in therespective semiconductor lasers, and a plurality of opticalsemiconductor portions integrally formed with said semiconductor lasersfor receiving the respective laser lights from said plurality ofsemiconductor lasers, wherein the refractive indexes of said diffractiongrating embedded semiconductor layers are decreased (or increased) so asto cancel the difference of the coupling coefficients of respectivesemiconductor lasers for the semiconductor lasers whose couplingcoefficient is increased (or decreased) when the diffraction gratingsfor generating a distributed feedback operation are formed in the sameconfiguration and the refractive indexes of the diffraction gratingembedded semiconductor layers are fixed at the same value.
 11. Amanufacturing method according to claim 10 , wherein said opticalsemiconductor integrated device comprises longitudinal single modeoscillating semiconductor lasers and optical modulators.
 12. Amanufacturing method according to claim 9 , wherein the band-gapwavelengths of said diffraction grating embedded semiconductor layersare made shorter (or longer) so as to cancel the difference of thecoupling coefficients of the respective semiconductor lasers for thesemiconductor lasers whose coupling coefficients are increased (ordecreased) when the diffraction gratings for generating a distributedfeedback operation are formed in the same configuration and the band-gapwavelengths of said diffraction grating embedded semiconductor layersare fixed at the same value.
 13. A manufacturing method according toclaim 12 , wherein said diffraction grating embedded semiconductor layeris made of InGaAsP, and the change of the refractive index of saidInGaAsP layer is executed by changing the compositional ratio of In andGa in Group III in the periodic table.
 14. A manufacturing methodaccording to claim 12 , wherein said diffraction grating embeddedsemiconductor layer is made of InGaAsP, and the change of the band-gapwavelength is executed by changing the compositional ratio of As and Pbelonging to Group V of the periodic table.
 15. A manufacturing methodaccording to claim 9 , wherein the method for changing the refractiveindex or the band-gap wavelength of said diffraction grating embeddedsemiconductor layer is a selective metal organic vapor phase growthmethod.
 16. A manufacturing method according to claim 9 , wherein themethod for changing the refractive index or the band-gap wavelength ofsaid diffraction grating embedded semiconductor layer is provided byadjusting a flowing ratio of Group V materials in an atmosphericpressure double fluid layer type metal organic vapor phase growthmethod.
 17. An optical semiconductor device according to claim 1 or thesemiconductor device obtained by the manufacturing method according toclaim 16 which is applied to an optical communication module, whereinthe optical communication module comprises: a waveguide device forguiding an output light from said optical semiconductor device to theoutside, a mechanism for inputting the output light from saidsemiconductor device to the waveguide device, and an electricalinterface for driving said semiconductor device.
 18. An opticalsemiconductor device according to claim 17 , wherein said semiconductordevice according to claim 1 or the semiconductor device obtained by themanufacturing method according to claim 16 is applied to an opticalcommunication apparatus, which comprises an optical transmission deviceprovided with said optical communication module and a receiving devicefor receiving the output light from said light transmission device.